Direct-coupled amplifier



Mrch 25, 1952 H. L. KING DIRECT-COUPLED AMPLIFIER Filed Nov. 5. 1950 (D N -u-- INVENTOR.

HOWARD L. KING.

Biz/1,50%

ATTORNEY Patented Mar. 25, 1952 DIRECT-COUPLED AMPLIFIER Howard L. King, Vancouver, Wash., assignor to the United States of America as represented by the Secretary of the Interior Application November 3, 1950, Serial No. 193,999 4 Claims. (01. 179-171) (Granted under the act of March 3, 1883, as amended April 30, 1928; 3'70 0. G. 757) The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to me of any royalty thereon in accordance with the provisions of the act of March 3, 1883 (22 Stat. 625), as amended by the act of April 30, 1928 (45 Stat. 467, 35 U. S. C., 1946 ed. sec. 45).

This invention relates to power amplifiers. It is concerned particularly with a type of amplifier capable of operating satisfactorily over a range of frequencies from zero to several thousand cycles per second. This invention is further of the kind known as direct-coupled amplifiers. A special characteristic of this invention is its adaptation to circuits which require an amplifier The grids of triodes 2 and 4 are excited by volt,-

ages of the same phase and magnitude from a of high input impedance while working into an output circuit of very low impedance. A principal application of this invention is in the operation of magnetic oscillographs whose moving elements are inherently of low impedance.

The principal objects of this invention include: amplification of currents of any frequency from zero to more than 10,000 cycles per second with very little phase shift; providing for amplification where input impedances of a megohm or more are required with output impedances of the order of one ohm; and providing for large output current capability in an amplifier with a minimum of harmonic current distortion. Other objects include: producing an amplifier capable of operating either with a balanced, push-pull,

or single ended input circuit;- providing for easy adjustment of output current over a wide range;

land providing an amplifier for high impedance input and low impedance output which is compact, portable and economical to construct. What constitutes this invention is set forth, with reference to the accompanying drawing, in the specification following, and is succinctly defined in the appended claims.

The drawing is a simplified circuit diagram of .my invention in a preferred form of embodiment. In the drawing the usual and conventional sources of current for cathode heaters in amplifying tubes and. other obviously understood conventional circuit components have been omitted from the diagram for convenience and brevity.

In the drawing a low impedance load I is connected into a balanced bridge circuit comprising thermionic triodes 2, 3, 4, and 5. Triodes 2 to 5, inclusive, are preferably of lowimpedance. The cathode follower action of these triodes makes the circuit adaptable primarily to low impedance output.

pair of triodes 6 and I. The grids of triodes 3 and 5 are excited by voltages opposite in polarity but equal in magnitude to those applied to the grids of triodes 2 and 4. Triodes 6 and 1 are driven by another pair of triodes 8 and 9 whose grids are excited. by an input circuit, usually of high impedance, comprising grounded resistors II and I2.

The output of triodes 9 and 9 is delivered to a pair of plate loading resistors I3 and H, in which the grid voltages for triodes 6 and 1 are produced. Triodes 6 and 'l are connected through their respective cathode circuits by a pair of resistors l5 and i6, connected at a common point I! to ground.

Two resistors l8 and I9 are provided in the plate leads of triodes 6 and I for providing the necessary plate-to-grid voltages of triodes 4 and 5. A variable resistor 2| is connected between the ground and the cathodes of triodes 2 and. 3 for control of plate current through triodes 2 to 5. A variable resistor 22 is connected between resistors I3 and I4 for compensating, by adjustment, the difference in characteristics of triodes 6 and 1 directly and, indirectly, other departures from symmetry in the characteristics Oct the circuit to provide a zero current balance across the load resistance I, under zero signal conditions. A grounded variable resistor 23 provides for cathode voltage drop for biasing triodes B and 9 through which the biases of the other tubes following are controlled, thus providing an adjustment for equal cathode-plate voltages across triodes 2 to 5 in the bridge.

In the operation of this amplifier, a signal is impressed on either resistor II or I2 or both in any convenient way. Push-pull signals are impressed at two terminals 24 and 25, or if singleended input is desired, the grid of triode 9 is grounded through a switch 26 and the signal applied between terminal 24 and ground. Operated with switch 26 closed, triodes 8 and 9 operate as a cathode-coupled phase inverter with a voltage other coupling devices. Triodes 6 and 1 deliver output to resistors I and I6 through their cathode connectors and to resistors l8 and I9 at the plates.

The principal feature of my invention is in the method of directly coupling the triodes 6 and 1 to the bridge of triodes 2, 3, 4 and 5. Triodes 6 and I each are connected as phase inverters. They together with resistors l5, I6, l8 and I9 which are of equal resistance provide equal si nal voltages to the grids of bridge triodes 2 to 5. Under condition of no signal, referred to also as the condition of static balance, the direct current voltage across resistors IE, IS, I8 and It are also equal, and it follows that the direct current grid-to-ground voltages of triodes 2 and 3 are equal to the direct current grid-to-plate voltages of triodes 4 and 5. Similarly the cathode-toplate voltages of all the triodes 2 to 5 must be the same in order to achieve linear push-pull operation. The plate currents in triodes 2 to 5 are the same for the no-signal condition, and accordingly the grid biases of all triodes 2 to 5 are the same. These equalities of voltages and currents are subject to some minor variations due to diiierences in tube characteristics. When the magnitudes of plate supply voltage and plate current are specified for the triodes 2 to 5 in the bridge of a particular amplifier, there is only one value of direct current voltage drop across resistors I5, I6, I8 and I9 that will meet the above stated requirements of equal pairs of voltages and currents.

When the input signal is zero, the equalities of voltages and currents stated above result in zero current in load I. An input signal, for example one that is positive on the grid of triode 8 in reference to the grid of triode 9, causes a decrease of voltage on the grid of triode 6, an increase of voltage on the grid of triode 5 and a decrease in voltage on the grid of triode 2. Similarly the grid of triode I is subjected to an increase in voltage, the grid of triode 4 to a decreased voltage and the grid of triode 3 to an increase in voltage. This produces a voltage difference across load and a current therein. A voltage impressed on the grids of triodes 8 and 9 in the direction opposite to that assumed above will produce a current in load I also in the 0pposite direction. The circuit as shown is stable partly because of the relatively low overall voltage gain, but primarily as a result of the pushpull arrangement throughout and the use of common cathode circuit resistors 2| and 23. These common cathode resistors provide self compensation which greatly decreases the need for matched pairs of tubes and which is relatively independ ent of voltage regulation thus minimizing the problem of drift inherent in all direct-coupled amplifiers.

This amplifier is adapted primarily to loads of low impedance such as the moving elements of oscillographs. The effect of increasing the load impedance is to cause grid-cathode degeneration with a consequent requirement of increase in grid driving voltage. The limit of load impedance is therefore determined by the manitude of grid driving voltage available and the limit of grid voltage permissible in the triodes 2 to 5. An increase in grid voltage to the point where the grids draw current introduces distortion in the output as compared with the input excitation.

Resistor 2| is capable of controlling the load current through a considerable range. Variation Of resistor 2| is capable of changing the current in load I by per cent with a change of only 5 per cent in the voltage drop through triodes 2 to 5, assuming no change in plate supply volt age. The push-pull arrangement of the bridge triodes 2 to 5 provides cancellation of the signal voltage across the common resistor 2|, and also if the load resistance is small the push-pull arrangement cancels the signal voltage at the load resistance I with respect to ground. Thus, even though there is a signal applied to the grids of the triodes 2 to 5 in the bridge there will be essentially only a direct current dilTerence of potential between load I and ground, and only a di rect current voltage across the common resistor 2|. This has the particular advantage of there being no grid-cathode degeneration except that provided by the load resistance, and this permits the maximum transconductance of each of triodes 2 to 5 to be realized. This also decreases any effect on frequency response of the amplifier caused by capacitance to ground of the load I and the wiring of the circuit. In actual practice there will be differences in tube characteristics and slight differences in applied signal voltage which will cause slight unbalances in the bridge but the push-pull arrangement of the bridge and common cathode resistor 2| provides self-compensation and extremely stable operation.

I claim:

1. In an amplifying circuit the combination of a bridge circuit comprising four electronic amplifiers connected in pairs the first pair of two connected together at the plates, and the second pair of two connected together at the cathodes, the pairs being connected with the cathodes of the first pair being respectively connected to the plates of the second pair, a source of plate current connected to the junction of the plates of said first pair and to the junction of the cathodes of said second pair, an output circuit connected between the two said cathode-plate junctions of the pairs, and means completely external of said bridge circuit for exciting the grids of said amplifiers so that the two grids of each pair are excited in opposite phase.

2. In an amplifying circuit the combination of a bridge circuit comprising four electronic ampli fiers connected in pairs the first pair of two connected together at the plates, and the second pair of two connected together at the cathodes, the pairs being connected with the cathodes of the first pair being respectively connected to the plates of the second pair, a source of plate current connected to the junction of the plates of said first pair and to the junction of the cathodes of said second pair, an output circuit connected between the two said cathode-plate junctions of the pairs, means for exciting the grids of said amplifiers so that the two grids of each pair are excited in opposite phase, a push-pull amplifying circuit for exciting the grids of said pairs, said push-pull amplifying circuit comprising a pair of electronic amplifiers with cathodes connected'together through resistors, the plates of said pushpull amplifiers being connected through resistors to the same source of plate current mentioned in claim 1, the plates of said push-pull amplifiers being further directly connected to the grids of said first pair of amplifiers insaid bridge, and the cathodes of said push-pull amplifiers being directly connected to the grids of said second pair of amplifiers.

3. In an amplifying circuitas described in claim 2, the combination thereof with a grid excitation circuit comprising a pair of electronic amplifiers connected like a push-pull amplifying circuit with the plates thereof directly connected to the grids of the push-pull amplifiers mentioned in claim 2, and to a pair of resistors connected at a common point to said source of plate current.

4. In an amplifying system, the combination of a bridge circuit comprising four electronic discharge devices connected .in pairs, each of said electronic discharge devices having at least a cathode, plate, and control electrode, the first pair of two connected together at the plates, and the second pair of two connected together at the cathodes, the pairs being connected with the cathodes of the first pair being respectively connected to the plates of the second pair, a source of positive voltage connected to the junction of the plates of said first pair and to the junction of the cathodes of said second pair, a load circuit being connected between the two cathode-plate junctions of the pairs, a third pair of electronic discharge devices each having at least a cathode, plate, and control electrodes, the cathodes of the third pair of electronic discharge devices being connected to a source of negative potential through a first and second resistor respectively, the plates of said third pair of electronic discharge devices being connected to said source of positive potential through third and fourth resistors respectively, said first, second, third and fourth resistors all being approximately equal in magnitude, a connection from the plate of each electronic discharge device of the third pair to the control electrode of a separate electronic discharge device of the first and second pair, a connection from the cathode of each of said electronic discharge devices of the third pair to a control grid of a separate electronic discharge device of said first and second pairs, said connection from the cathode and plate of either electronic discharge device of the third pair being to the control grids of tubes of the first and second pairs connected to the same plate-cathode junction, and said grid connection from either of the first or second pair being connected to corresponding electrodes of the third pair of electronic discharge devices, a push-pull signal voltage source being connected between the grids of the third pair of electronic amplifiers.

HOWARD L. KING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,329,073 Mitchel et a1 Sept. '7, 1943 2,428,295 S cantlebury Sept. 30, 1947 

